두 갈래 트리 앞 순서, 중간 순서, 뒤 순서 반복 (귀속)

/**
 * Definition of TreeNode:
 * class TreeNode {
 * public:
 *     int val;
 *     TreeNode *left, *right;
 *     TreeNode(int val) {
 *         this->val = val;
 *         this->left = this->right = NULL;
 *     }
 * }
 */

class Solution {
public:
    /**
     * @param root: The root of binary tree.
     * @return: Preorder in vector which contains node values.
     */
     void pretrav(TreeNode *root,vector<int> &r){
        if(root==NULL) return;
        r.push_back(root->val);
        pretrav(root->left,r);
        pretrav(root->right,r);
    }
    vector<int> preorderTraversal(TreeNode *root) {
        // write your code here
        vector<int> v;

       pretrav(root,v);
       return v;

    }

};

void intrav(TreeNode *root,vector<int> &r){
        if(root==NULL) return;
        intrav(root->left,r);
        r.push_back(root->val);
        intrav(root->right,r);
    }
    vector<int> inorderTraversal(TreeNode *root) {
        // write your code here
        vector<int> v;
        intrav(root,v);
        return v;
    }

void posttrav(TreeNode *root,vector<int> &r){
        if(root==NULL) return;
        posttrav(root->left,r);
        posttrav(root->right,r);
        r.push_back(root->val);
    }
    vector<int> postorderTraversal(TreeNode *root) {
        // write your code here
        vector<int> v;
        posttrav(root,v);
        return v;
    }